This invention relates in general to electric arc furnaces and in particular to an apparatus and method for power control in such a furnace.
Alternating current (AC) electric arc furnaces are commonly used to melt or smelt solid materials, such as metals or ore bearing materials. Such furnaces generally use high power arcs to generate heat energy in a refractory lined vessel, and include a power supply for controlling the electrical energy supplied to the arc. High power arcs are an energy conversion mechanism that behave as a non-linear time-varying impedance. Consequently, the voltage, current and power drawn by an arc furnace tends to fluctuate, causing disturbances to both the melting/smelting process and to the supply network. These disturbances can result in inefficiencies, increased equipment wear, and in extreme cases damage to the supply network or arc furnace.
Various attempts have been made to regulate arc furnace power supplies. For example, in some arc furnaces a fixed series reactor has been used to deliver modest improvements in arc stability. Static Watt Compensators (SWC), consisting of a shunt connected thyristor switched resistor bank, have been used to mitigate load rejections. Electrode regulators, which control arc electrode movement, have been used to regulate the relative position of the arc electrodes in order to maintain a set point electrode voltage, current or impedance.
Some control systems have been directed primarily towards stabilizing voltage in an arc furnace. For example, a Static VAR Compensator (SVC) consists of a shunt connected harmonic filter bank and a shunt connected thyristor-controlled reactor, which operate in concert to lower voltage flicker or maintain a constant furnace power factor. The SVC operates by shunt injection of either capacitive or inductive reactive power, thereby maintaining a constant voltage by maintaining the total reactive power draw (MVAR) of the furnace balanced near zero (ie. neither inductive or capacitive).
Other control systems have been primarily directed towards stabilizing current in an arc furnace. For example, U.S. Pat. No. 5,239,554 issued Aug. 24, 1993, to Gensini et al. discloses regulating arc current through the use of controlled series reactances, consisting of a series connected saturable reactor, or a series connected thyristor switched reactor. U.S. Pat. No. 5,991,327 issued Nov. 23, 1999, to Kojori discloses a controller that uses predictive software to gate a thyristor assembly installed in series with the arc in order to supress current swings which cause voltage flicker.
As power is the product of voltage and current (P=Vrms*Irms*Power Factor), arc furnace power supplies that operate to stabilize either current or voltage permit the power draw of the arc furnace to fluctuate extensively. In larger furnaces, the active power draw can change by tens of Mega Watts (MW) within short time spans. In many industrial sites in the world, islanded power stations (isolated from a utility grid) supply power to relatively large arc furnaces. Furnace power fluctuations can result in frequency/speed fluctuations in rotating generating equipment, which, for example, may be steam turbines, diesel powered pistons, or water driven turbines. The power generating equipment has upper and lower limits to the frequency fluctuations that can be absorbed without resulting in mechanical damage. Beyond such limits, mechanical and electrical damage can occur. Furthermore, even if immediate damage does not occur, ongoing power and frequency fluctuations cause increased wear and tear on the generating station. To date, these frequency swings have been compensated for by installing bypass valves for water or steam release at the generating station (in the case of hydro or steam turbines), adding additional rotating inertia to dampen the frequency swings, and oversizing the generating station. Such solutions tend to be expensive and inefficient.
Stable constant power in an electric furnace provides for an accurate balancing of power to feed material, which in turn maximizes the energy efficiency of the furnace through efficient heat transfer thus facilitating a high furnace throughput level. Accordingly, there is a need for an efficient, cost effective power control system for use in arc furnaces. There is also a need for a power control system which reduces the magnitude and frequency of power fluctuations in an arc furnace.
According to the present invention, a power supply control system for an electric arc furnace uses variable reactor control and electrode height regulation to regulate the power used in an arc furnace so as to reduce active power fluctuations. In general, the arc furnace control system of the present invention operates by selecting a furnace transformer voltage tap to match a furnace power set-point, continuously adjusting variable reactors to regulate power swings on a cycle by cycle basis to counter changes in electrode impedance, and mitigating power dips and rises by predictive electrode height regulation.
According to one aspect of the invention, there is provided a power control system for an AC electric arc furnace having an AC power source for applying power to an arc electrode, in which the power control system includes a variable reactance intermediate the power source and the electrode, and a variable reactor controller for monitoring an impedance of the electrode and causing the variable reactance to vary in response to changes in the monitored electrode impedance so as to reduce variations in the active power provided to the electrode. Preferably, the power control system includes an electrode movement device for adjusting the electrode height, and an electrode position controller configured to predict the possible onset of loss of arc for the electrode. If loss of arc is predicted, the electrode position controller causes the electrode movement device to rapidly lower the electrode and the variable reactor controller to momentarily reduce the magnitude of the variable reactance.
According to another aspect of the invention, there is provided a power control system for an AC electric arc furnace having an AC power source for applying power to an arc electrode, including an arc electrode movement device for adjusting the height of the electrode to control the arc length thereof, and an electrode position controller for controlling the operation of the electrode movement device, the position controller being configured to monitor operating characteristics of the arc furnace to predict the onset of a plurality of upset conditions and cause the electrode height to be adjusted in response to a specific predicted upset condition.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.